1. Field of the Invention
The present invention relates generally to fuel assemblies for a nuclear reactor and, more particularly, is concerned with fuel rods in a fuel assembly containing axial regions of annular and standard fuel pellets having the same U-235 enrichment.
2. Description of the Prior Art
In a typical pressurized water nuclear reactor (PWR), the reactor core includes a large number of fuel assemblies each of which is composed of a plurality of elongated fuel elements or rods transversely spaced apart from one another. The fuel rods each contain fissile material and are grouped together in an array organized to provide a neutron flux in the core sufficient to support a high rate of nuclear fission and thus the release of a large amount of energy in the form of heat. A liquid coolant is pumped upwardly through the core in order to extract some of the heat generated in the core for the production of useful work.
The fissile material contained in each fuel rod is typically enriched uranium (for example, uranium which has 4.0 weight percent U-235 in U-238), provided in the form of a stack of fuel pellets. The reaction which produces energy in a nuclear reactor is the neutron-induced fission of enriched uranium atoms. However, neutrons released at the opposite ends of the fuel stack have a greater probability of escaping from the core than do neutrons in the portion of the fuel rod between the ends. Because these escaped or leaked neutrons can no longer produce fission events, neutron leakage is wasteful, expensive and should be reduced.
Due to the escape of a greater proportion of neutrons at the opposite ends of the fuel rod, the distribution of fission-inducing neutrons is approximately cosinosoidal along the axis of the fuel rods. As a result, the depletion of fissile fuel, or burnup, distribution along the length of a fuel rod is non-uniform, with the ends of the rod receiving less burnup than the center. This is inefficient utilization of the enriched uranium in the fuel rod ends. Since more than 70% of the total fuel cycle cost is devoted to buying and enriching uranium to ensure that there is enough available to maintain the fission chain reaction, approaches which can reduce the number of neutrons that escape from the core produce significant economic benefits.
One well-known approach to reducing neutron leakage and this resulting inefficiency is to create axial blankets at the top and bottom of the fuel rods. Axial blankets are created by substitution of pellets of natural uranium (i.e., uranium which has 0.71 weight percent U-235 in U-238) for pellets of enriched uranium in short regions at both ends of the fuel rod. In the VANTAGE+PWR fuel rods manufactured and marketed by the assignee of the present invention, axial blanket pellets have typically solid right cylinder configurations, although optionally annular axial blanket pellets are available for use, if necessary, to provide more space for fission gas release. By way of example, in the VANTAGE+PWR fuel assembly each fuel rod is 144 inches long, each region of axial blanket natural uranium pellets at each opposite end is 6 inches long and the remainder of the pellet stack of enriched uranium extending between the axial blankets is 132 inches in length. Since these axial blankets contain less uranium 235, they give up fewer neutrons to leakage from the core.
Axial blankets reduce neutron leakage approximately 50% and function to generate plutonium by the absorption of neutrons. Because plutonium is fissile material, the blanket pellets improve the burnup distribution somewhat. However, axial blankets release fewer neutrons at the beginning of life (BOL) than toward the end of life (EOL) of the core when more plutonium is generated. This means that there is more neutron leakage in the case of axial blankets at EOL than at BOL which is just the opposite of what is desirable. Thus, even fuel rods using axial blankets at their opposite ends fail to achieve full burnup and thus attain less than optimum fuel utilization at such locations.
Another approach disclosed in U.S. Pat. No. 4,493,814 to Beard, Jr. et al, assigned to the assignee of the present invention, is to substitute in place the pellets in an inner axial region of each axial blanket and in an outer axial region of each opposite end of the remaining standard pellets a row of low density fuel pellets. The remaining pellets between the low density pellets can be standard density fuel pellets. For example, now the axial blankets will each be 4 inches in length, each row of low density pellets will be 4 inches long and the remaining standard pellets in between will be 128 inches in length. These low density pellets could be hollow annular pellets, lower density pellets, smaller pellets or some other concept designed to reduce the uranium loading per unit length. The uranium 235 enrichment of these low density pellets would typically be the same as the standard enriched uranium pellets of the rod.
This latter approach is believed to improve neutron flux distribution and consequently the resulting burnup distribution and utilization of uranium over the use of axial blankets alone. While provision of the low density pellets in Beard, Jr. et al appears to be a step in the right direction, the approach still fails to reach optimum results in terms of uranium burnup and utilization. Still another approach disclosed in U.S. Pat. No. 4,687,629 to Mildrum, assigned to the assignee of the present invention, is to provide a fuel rod with a complete stack of annular fuel pellets having the same U-235 enrichment and different annulus sizes for graduated enrichment loading. This approach designed with a boiling water nuclear reactor (BWR) fuel rod in mind is not seen as providing an optimum design for uranium burnup and utilization in a PWR fuel rod. Consequently, a need exists for further improvements which hold out promise to provide optimum results.